An artificial hip joint comprises a femoral component which is essentially a ball mounted at the end of the femur and an acetabular component which is a socket mounted in the hip bone. Hip prosthesis failure is most commonly associated with the loosening of one or both of these components. Loosening alone may be symptomatic in various degrees and may, in itself, be an indication of the necessity of revision or reoperation to correct the problem. Sometimes, however, such loosening may have drastic consequences such as protrusion of the acetabular or proximal femur fracture. Such prosthesis failure can be traced ultimately to two problems. Either the tissue reacts to the prosthetic materials used or the tissue reacts to grossly altered mechanical properties of the prosthesis from the natural bone.
More particularly, the joint components are foreign to the bone tissue and cause some tissue necrosis at the interface with the prosthesis. More damaging to the bone tissue, however, is the polymethylmethacrylate (PMMA) cement which is usually used to fix many conventional prostheses in the bone structure. That cement is a self-polymerizing synthetic plastic whose polymerization gemerates temperatures which are high enough to damage surrounding bone tissue. Moreover, the monomer component of that cement is a toxic substance. Unless there is complete polymerization of the cement components, unbound catalyst monomer remains which can leak into the surrounding bone tissue. In fact, such leakage has already been identified as a major source of PMMA toxicity in patients receiving artificial hip joints.
In order to avoid such problems occasioned by the use of cements, hip joint prostheses have been devised which do not require such cement fixation. Examples of some such prostheses not requiring cement are disclosed, for example, in U.S. Pat. Nos. 4,051,559; 4,080,666; 4,101,985 and 4,187,559.
Prior conventional prostheses, however, do not completely address the loosening phenomenon due to the grossly altered mechanical properties of the implant and the nonphysiological nature of the application of force from the implant to the bone. One good exposition of that problem is contained in the aforementioned U.S. Pat. No. 4,187,559. However, the femoral prosthesis described in that reference is not completely satisfactory because it does not take into account the entire structural nature of the bone receiving the implant with the result that the implant is not really the mechanical equivalent of its natural counterpart. Consequently, that prior implant creates unnatural stresses in the remaining bone structure which tends to cause long-term deterioration of the bone and loosening of the implant. A brief explanation of the structure of the femur will now be given with reference to FIGS. 1, 2 and 2A of the accompanying drawings.
FIG. 1 illustrates the internal structure of the proximal femur (see Color Atlas of Human Anatomy by R. M. McMinn and R. T. Hutchings, page 275). The shaft S branches into a head which is the ball component of the natural hip joint and the so-called greater trochanter which is a protrusion on the lateral side of the femur. The major groups of structural units of the cancellous bone (trabeculae) are identified in that figure as follows:
(A) those from the medial surface of the shaft to the head; PA0 (B) those from the lateral surface of the shaft to the head; PA0 (C) those from the medial surface of the shaft to the greater trochanter; PA0 (D) those from the lateral surface of the shaft to the greater trochanter; PA0 (E) the calcar femorale which is a dense concentration of trabeculae extending from the region of the lesser trochanter to the undersurface of the neck; PA0 (G) a triangular area of few trabeculae; and PA0 (H) the region of attachment of trabeculae groups B and D to the lateral side of the femur shaft.
FIG. 2 illustrates a simplified and useful structural analogy to the bone structure at the upper end of the femur illustrated in FIG. 1. The system is statically determinate and the reactions to the applied force F follow from the vector diagram shown in FIG. 2A. Truss elements A and C which correspond to the femur trabeculae A and C described above are loaded in compression, while cables B corresponding to trabeculae B are loaded in tension. Reaction forces F.sub.E and F.sub.G are transferred to the support S corresponding to the femur stem S. From these figures, it is clear that the medial side of the femur cortical shell will be loaded in compression, while its lateral side will be loaded in tension. Force F.sub.T which is applied in nature by muscles attached to the greater trochanter and through trabeculae D will add to the tensile loading of the lateral side of the femur as well as to compressive loading of trabeculae C.
The designs of commercially available femoral prostheses are very similar. They all comprise a ball or head connected by way of a neck to a long stem or shaft which is inserted deep into the femoral intramedullary canal to support the head at the proximal end of the femur. The variations from one prosthesis to another are related primarily to the choice of stem design. In all of these designs, including the one described in the above-mentioned U.S. Pat. No. 4,187,559, the long metal stem of the prosthesis is orders of magnitude stiffer than the replaced material, i.e. the soft cancellous bone and marrow of the intramedullary canal. Therefore, the stress patterns in the upper femur are significantly altered by the implant. Also, under load, a force couple is imparted to the femur which is transmitted to the opposite walls of the cortical tube in directions perpendicular to the trabeculae. These forces do not promote bone growth and unduly stress the walls of the femur, sometimes causing fractures thereof.
The concept of using a rigid stem inserted deep into the intramedullary canal to transfer loads from the artificial joint to the femur is inherently destructive to the bone and probably arose initially due to the appealing simplicity of prosthesis insertion. However the presence of the rigid prosthesis stem in the femur causes drastic changes in the stress distributions in the altered bone which strongly prejudice long-term success of the joint replacement. While improvement of component materials and the design of such implants has more or less solved the problem of prosthesis stem fracture, stem loosening and bone fracture have surfaced as the major long-term complications inherent in hip joint replacement.